This invention relates to seismic surveying as used in oil and gas exploration. This commonly involves the use of vehicles carrying earth shock devices (e.g. impact or vibration devices) which are required to move forwards step by step and to make impacts or vibrations on the ground at predetermined intervals of length. It is well known to mark out a length of territory by wooden pegs or similar markers at say 30 meter intervals and within each pair of adjacent pegs it is required to effect several impacts or vibrations which may be at equal or unequal distances apart according to a predetermined pattern. The drivers of such vehicles however have considerable difficulty in stopping the vehicles accurately at the required positions. Because of this difficultly one constant move up distance is employed between each impact or vibration for all the vehicles. This limits the variety of predetermined patterns which can be employed.